1. Field of the Invention
The present invention relates to method and device for lighting ultra-high pressure discharge lamps capable of minimizing deposition of metallic mercury on electrode surfaces, ensuring early stabilization of arc, preventing blackening, and avoiding the formation of mercury bridge at least between tip ends of the electrodes.
2. Description of the Related Art
Recently, ultra-high pressure discharge lamps are frequently used as light sources in information systems such as a liquid crystal projector. Acute competition exists in pursuit of an ultra-high pressure discharge lamp that is capable of providing a sharper and brighter picture, being used as a smaller point light source, offering a higher luminance, and enjoying a longer lifetime for use as a light source in liquid crystal projectors in particular. To meet such demands, the internal volume of an arc tube forming part of an envelope has been gradually reduced. Quite recently, an arc tube having an internal volume as small as about a half of a typical tube has been developed. With this downsizing trend, the spacing between electrodes becomes very narrow or as small as 1.5 to 1 mm. On the other hand, the amount of mercury filled in such an arc tube per unit volume of the tube has been largely increased. Quite recently, the amount of filled mercury has become about twice as large as the typical amount. For this reason, in an extreme case, metallic mercury that has condensed on an electrode surface upon extinction of the lamp is evaporated by absorbing the heat of arc in initiating lighting of the lamp, with the result that the temperature of the electrodes is prevented from rising thereby hindering the formation of a hot arc spot, hence causing arc to break off. Another problem associated with the downsizing trend is that a mercury drop condensed upon extinction of the lamp extends between the electrodes to form a mercury bridge which short-circuits the lamp, thereby hindering the lamp from lighting.
The condensation of metallic mercury on electrode surfaces and the formation of a mercury bridge resulting from growth of condensed metallic mercury are considered to occur as follows. Typically, an ultra-high pressure discharge lamp is used as attached to a concave reflecting mirror. The ultra-high pressure discharge lamp attached to the concave reflecting mirror is cooled by direct blowing into the concave reflecting mirror or to the lamp-receiving portion of the mirror. Then, at least one of the pair of electrodes of the ultra-high pressure discharge lamp is cooled faster than mercury vapor still kept at a high temperature in the arc tube. The mercury vapor is then deposited and condensed on the electrode thus cooled, and the condensed mercury gradually grows into a mercury drop, which in turn flows into the narrow spacing between the electrodes to form a mercury bridge.
Even if such a mercury bridge is not formed, a large amount of mercury is deposited on the electrode surface as described above so that arc generated from the deposited mercury in initiating lighting of the lamp moves unstably on the electrode surface until the deposited mercury has been thoroughly evaporated. Particularly where the ultra-high pressure discharge lamp is an AC discharge lamp adapted to start lighting with direct current in an early lighting stage (0.5 to 5 seconds) and thereafter to light with alternating current of low frequency, the cathode is hard to heat and is derived of heat by mercury if deposited in a large amount thereon so that arc is likely to break off. This tendency is conspicuous when air-cooling is adopted as described above. It is conceivable to prolong the high-voltage-generating period in order to prevent arc from breaking off. However, this cannot be said to be desirable in terms of safety. Further, if the time period for which arc is instable as described above is prolonged, the sputtering action of arc causes the electrode material to scatter and adhere to the inner surface of the arc tube, thus causing a blackening phenomenon. FIG. 8 is a block diagram of a prior art lamp lighting circuit, and FIG. 9 is a time chart of the operation of this lamp lighting circuit. As shown, the lamp lighting circuit includes an igniter section 30 for applying high voltage pulses in initiating lighting of an ultra-high pressure 1, a stabilized lighting circuit 31 for stabilized supply of a lighting power to the ultra-high pressure discharge lamp 1 during a steady lighting stage, and a power control section 32 for controlling the stabilized lighting circuit 31. Upon receipt of a lamp lighting control signal (lamp extinguishing signal), the ultra-high pressure discharge lamp 1 is extinguished and then the deposition of mercury or the formation of a mercury bridge proceeds as described above.
Accordingly, it is an object of the present invention to improve the lighting performance of an ultra-high pressure discharge lamp by minimizing condensation of mercury on electrode surfaces following extinction of the lamp. Specifically, the object of the present invention is to provide method and device for lighting an ultra-high pressure discharge lamp capable of stabilizing arc in a shorter time and preventing the occurrence of blackening and the formation of a mercury bridge.
In accordance with a first aspect of the present invention, there is provided a method of lighting an ultra-high pressure discharge lamp in which a pair of electrodes are disposed confronting each other with a spacing of not more than 1.5 mm therebetween in an arc tube forming part of an envelope of quartz glass, the arc tube encapsulating mercury in an amount of 0.15 mg/mm3, the method comprising the steps of:
reducing a lamp power supplied to the pair of electrodes to a degree such as not to stop arc discharge in a transition state from a lighting state to extinction;
keeping the lamp power thus reduced for a predetermined time period; and
shutting down the supply of current to the electrodes.
With this method, feeble arc is generated between the electrodes in the transition state from a lighting state to extinction. For this reason, the temperature of the electrodes is kept higher than the evaporating temperature of mercury and, hence, mercury vapor does not condense even when contacting the surfaces of the electrodes. Since the envelope is under cooling, mercury vapor contacting the inner surface of the arc tube condenses and gradually grows thereon while gradually reducing the pressure of mercury vapor within the arc tube.
When the supply of current to the electrodes is shut down at the time the mercury vapor pressure within the arc tube has lowered sufficiently, residual mercury vapor, the amount of which is very small, starts condensing. Since the residual mercury vapor preferentially condenses on the arc tube already cooled rather than on the electrodes just finished with arc discharge and hence still in a heated state, condensation of mercury on the electrodes is limited. As a result, a mercury bridge is not formed at all.
Further, since the deposition of mercury on the electrodes is very little, the small amount of mercury on the electrodes serving as a starting point of arc generated between the electrodes in initiating re-lighting of the lamp is evaporated in a short time and, hence, the arc moves to between the confronting ends of the electrodes and is maintained stably thereat. Accordingly, the arc moving period in the initiating stage is very short, so that the occurrence of blackening due to sputtering during the arc moving period in the initiating stage is restrained, thus contributing to an improvement in the lifetime of the lamp.
In one embodiment, the lamp power is reduced to a value xc2xd to {fraction (1/20)} times as large as a rated output of the discharge lamp.
In another embodiment, the time period of keeping the reduced lamp power is 1 to 20 seconds.
In accordance with another aspect of the present invention, there is provided a device for lighting an ultra-high pressure discharge lamp, comprising: an igniter for initiating lighting of the ultra-high pressure discharge lamp by applying pulses of a high voltage thereto; a stabilized lighting circuit connected to the igniter for causing the ultra-high pressure discharge lamp to perform stabilized lighting; and a power control section for controlling power supply from the stabilized lighting circuit to the ultra-high pressure discharge lamp,
the power control section having a lamp power output reduction control function which serves to control the stabilized lighting circuit so that a lighting power is stably supplied from the stabilized lighting circuit to the high-pressure discharge lamp in a steady lighting stage while controlling the stabilized lighting circuit so that a power outputted to the ultra-high pressure discharge lamp is reduced to a lamp power such as not to stop arc discharge between a pair of electrodes in a transition state from steady lighting to extinction of the ultra-high pressure discharge lamp.
These and other objects, features and attendant advantages of the present invention will become apparent from the reading of the following detailed description of the invention in conjunction with the accompanying drawings.